Engines may use various forms of fuel delivery to provide a desired amount of fuel for combustion in each cylinder. One type of fuel delivery uses a port injector for each cylinder to deliver fuel to respective cylinders. Still another type of fuel delivery uses a direct injector for each cylinder.
Engines have also been described using more than one injector to provide fuel to a single cylinder in an attempt to improve engine performance. Specifically, in US 2005/0155578 an engine is described using a port fuel injector and a direct injector in each cylinder of the engine. In this system, transient errors in delivered fuel from the port injector, such as due to sudden airflow increases, can be addressed by subsequently delivering fuel of the same type from a direct cylinder injector.
However, the inventors herein have recognized a disadvantage with such an approach. Under such transient conditions where sudden load increases may occur, engine knock may be more likely. As such, even if the air-fuel ratio is correctly maintained, engine knock may still occur, thus reducing performance and driver satisfaction.
Another approach that utilizes multiple injection locations for different fuel types to reduce knock is described in the papers titled “Calculations of Knock Suppression in Highly Turbocharged Gasoline/Ethanol Engines Using Direct Ethanol Injection” and “Direct Injection Ethanol Boosted Gasoline Engine: Biofuel Leveraging for Cost Effective Reduction of Oil Dependence and CO2 Emissions” by Heywood et al. Specifically, the Heywood et al. papers describe directly injecting ethanol to improve charge cooling effects under various conditions, such as in response to a knock sensor or steady state operating conditions.
However, the inventors herein have recognized that transient engine knock may still occur during transient conditions, such as aggressive driver tip-ins, since the feedback information from the sensor become available too late to correct the combustion mixture during the transient and reduce knock. As such, performance and satisfaction may still be degraded.
Therefore, in one approach, a method of controlling an engine, the method comprising: providing fuel having a blend to a cylinder of the engine; actively varying said fuel blend in response to at least an operating condition; where during a transient operating condition, said blend is adjusted to increase a heat capacity of said fuel to reduce a tendency for knock.
In this way, it is possible to adjust the blend before the onset of knock, and thus improve performance. For example, it may be possible to continue engine boosting during the transient to thereby provide improved vehicle performance.